1. Field of the Invention
This invention pertains to the collection of seismic data related to a generally known seismic formation having a significant dip from a central location. An example of specific geologic structure of interest is a salt dome structure.
2. Background of the Invention
Seismic data collection procedures in the prior art have normally been about the same for all types of geologic formations even though in many instances there is prior knowledge of the geologic formation about which more detailed knowledge is desired. Normally, general foreknowledge regarding the target is taken into account by the survey line spacing, the shotpoint spacing, and the receiver location spacing. When the formation is suspected of including rather steep dips, then the receiver locations are closer to one another and the survey lines will be drawn closer together than for a generally flat terrain.
Receiver spacing and survey line spacing are adjusted so as to avoid aliasing and to ensure that the seismic data migrates to its proper location. Steep dip leads to close spacing requirements.
The current procedure begins typically with mapping off an area containing the geologic formation to be explored. Parallel survey lines are established across the area. Some of these lines will be `dip` lines while many more may often be `strike` lines.
In addition to establishing survey lines, the area is also divided up into so-called rectilinear "bins", which are rectangular grid areas, into which collected data is assigned for subsequent data processing. When the survey lines, shotpoints, and receiver locations are such that a specified amount of data can be assigned to a particular bin, the data within that bin is deemed statistically valid. Shotpoints are positioned to provide quality data in the bins. When there is steep dip in the subsurface, there can be bins with little data assigned thereto if the survey lines, shotpoints, and receiver locations are too far apart. Hence, there is an additional need for close spacing. An enormous amount of data is normally collected and processed. Even for an area of just a few square miles, a large amount of data results for processing. Such data gathering and data processing effort is extremely expensive.
Shotpoints of a typical survey in the prior art are located at regular intervals, for example, every 15 meters even at a distance from the formation, which may be as great as five or more miles (approximately 8 Km). The mid-point reflection points are anticipated to be approximately halfway to the receiver locations on the survey line. For some surveys, receivers are simultaneously located on survey lines other than the ones coincident with the corresponding shotpoints. In the taking of the data, shots are initiated sequentially along a shotline while all the receivers simultaneously record seismic data. Because of the distances and directions involved to the geologic formation, much of the data will have relatively large amounts of noise along with the signals received off the target.
When a shotpoint and receiver location are on a survey line that crosses the dip along a "strike" line, a reflection from their common midpoint is directed off sideways at an angle. This can yield data with a poor signal-to-noise ratio. In addition, closely spaced survey lines are required so that this data remains unaliased when 3-D migration is applied to image the geologic structure. However, when the shotpoint and receiver locations are on a survey line that is also a dip line, the reflection returns to a receiver location on the survey line. This will be a stronger return providing useful data. If all the survey lines could be dip lines then migrating each of these radial 2-D lines negates the need for applying conventional 3-D migration that is required when conventional parallel survey lines are used.
Therefore, it is a feature of the present invention to capitalize upon previous knowledge concerning the symmetry of a geologic formation under investigation and to collect mostly meaningful data when it is known that steep dip is present and to coordinate radial collection with respect thereto.
It is another feature of the present invention to provide an improved method of seismic data collection with respect to suspected steep dip formations such that the 2-D seismic data is all shot along dip lines so that the collected data is higher quality compared with conventional methods.
It is another feature of this present invention to provide an improved seismic data collection procedure including establishing and migrating resulting 2-D radial lines in such a manner so as to eliminate the need for applying conventional 3-D migration.